Differences in the interfacial reaction between Cu substrate and metastable supercooled liquid Sn-Cu solder or solid Sn-Cu solder at 222 degree C: Experimental results versus theoretical model calculations

Interfacial reactions between a Cu substrate and a metastable supercooled liquid Sn-Cu solder at 222 degree C are studied for the first time. The principal aim of this study is to compare the differences between Cu/solid Sn and Cu/metastable liquid Sn interfacial reactions at a rigorously identical temperature and to determine the role of liquid state in the kinetics of interfacial reactions as well as in the morphology of the reaction product. For this purpose, we have performed specific experiments for the interfacial reaction between metastable liquid Sn-0.7wt%Cu alloy and Cu substrate at a temperature of 222 degree C, that is 5K lower than the melting point of this alloy (227 degree C), and for reaction times as long as 32h. These experiments have been carried out using a Differential Scanning Calorimetry apparatus in order to monitor and control the physical state of the alloy as well as to set the accurate reaction temperature. Similar experiments were performed to study the reaction kinetics between the Cu substrate and solid Sn-0.7wt%Cu alloy at the same temperature. A large difference in the growth kinetics and the morphology of the eta -Cu6Sn5 phase was observed when a change in the physical state of the alloy occurs at identical temperature. Using thermodynamic and kinetic models of growth kinetics of the reaction layer at reactive interfaces, the driving force of eta -Cu6Sn5 formation cannot be the cause of this large difference. A theoretical analysis of the growth kinetics of the eta -Cu6Sn5 phase and comparison of its growth in both configurations are compared.